In many engineering applications, a surface situated in a flow field is geometrically configured so that the flow induces a force on the surface; hence these configurations are called lifting surfaces. The most common examples are airplane wings, vanes on turbines and wind mills, and helicopter rotors. Under many operating conditions, the velocity near the lifting surface is approximately parallel to the surface and in the same direction as the mean flow field. When this is not true, regions of separated flow exist which are characterized by at least one point where a significant velocity component perpendicular to the surface exists. The present invention applies to these regions, but for simplicity the particular geometrical configuration discussed below is the delt wing. Similar applications exist for other geometries.
A delta wing 100 have a semi-vertex angle B and an angle of attack A is sketched in FIG. 1. Under usual operating conditions, the lift on such wings is due primarily to the large bound vortices (LBV) 101 which appear on the low pressure side of the wing. With straight line leading edges, only two LBVs will occur, however, more may be created by discontinuities in the leading edge or in the velocity along the edge. The LBVs are associated with a shear layer originating near the leading edge at the point of separation. This shear layer surrounds the vortices and may be considered as an integral part of the vortices 101 as shown in FIG. 2 which is a Section through 2--2. The sense of rotation of the vortices 101 is also shown. The strength and size of the shear layer and vortices 101 depends upon the velocity, angle of attack, swept back angle, configuration of the leading edge, etc. The lift on the wing depends, to a large extent, on the strength of these vortices 101.
Recent observations have shown that the shear layer in FIG. 2 is also composed of discrete vortices 103 which originate near the leading edge. They form at a discrete frequency dependent upon the mean velocity and the parameters effecting the thickness of the shear layer, and they grow by pairing with each other as they move downstream, much the same as in a mixing layer. The LBV 101 may be described as being composed of the collection of the individual vortices 103.
In lifting surfaces, it is desirable to control the lift of the surface for a number of reasons. For example, a fighter plane with variable lift would be able to attain an unusual degree of maneuverability. A number of devices have been proposed to control the lift of lifting surfaces by altering the large bound vortices 101. Such an approach requires the use of relatively large control surfaces such as flaps on the leading edge. Such devices have not met with wide spread adoption, however. It would be desirable to control lift by altering the discrete vortices 103.
A number of devices have been proposed to control the shedding of vortices at the leading edges of wings and other lifting surfaces. These include wings upstream of the leading edge, and various tabs and slots on the leading edge. None of these devices control the discrete vortex formation as a means of controlling bound vortices.
It has also been proposed to insert tuned cavities in the wing surface to control boundary layers, reduce noise or prevent stalling. Such devices are not intended to control discrete vortices. These devices have not met with commercial adoption.